The present invention relates to low molecular weight high-cis polybutadienes synthesized with a catalyst system that comprises a neodymium-containing compound, an organoaluminum hydride, and a halide source in combination with either an alkyl aluminoxane or a trialkyl aluminum compound. Blends of these low molecular weight high-cis polybutadienes and high molecular weight high-cis polybutadienes are further disclosed for use in tire tread compounds to enhance fracture properties, snow traction, wet traction, and rolling resistance of a tire.
In the past, low molecular weight polybutadienes have generally been prepared using Nickel- and Cobalt-based Ziegler catalysts with specific molecular weight regulators. These molecular weight regulators have been problematic. More particularly, it is generally accepted that these molecular weight regulators broaden the molecular weight distribution or reduce the rate of polymerization. For example, as shown in an article in Bull. Chem. Soc. Jpn., 65, pp.1307-1312 (1992) at Tables 3 and 4, the use of these molecular weight regulators in combination with co-ordination catalysts has in the past generally resulted in a decrease in the cis-1,4 content.
Japanese Patent Publication 8-73515 discloses the polymerization of dienes with Group 3B metal (including neodymium) catalyst systems, wherein molecular weight regulators are not used in the polymerization process. The methods taught therein are applicable to a situation for making a high molecular weight high-cis polybutadiene with a narrow molecular weight distribution. But the method taught therein, as shown in the examples, generally produced a very broad molecular weight distribution if used to form a low molecular weight high-cis polybutadiene.
More particularly, while the data in that application indicated that narrow molecular weight distributions were possible at number average molecular weights of 250,000 and above (see Table 1 Practical Examples 1-5), all of the examples producing number average molecular weights below 150,000 resulted in broad molecular weight distributions (e.g. 5.5-6.3). Molecular weight distributions are the weight average molecular weight divided by the number average molecular weight. Narrow molecular weight distributions are generally achieved by controlling the initiation of polymer chains, termination of polymer chains and the lifetime of each polymer chain so that each and every polymer chain grows for the same period of time and consequently grows to approximately the same molecular weight.
Moreover, it is known that low molecular weight polybutadienes can be blended with high molecular weight high-cis polybutadienes. These known low molecular weight polybutadienes, however, having a broad molecular weight distribution and/or a reduced cis-1,4 content, cannot be readily used in these blends. For example, because of the broad molecular weight distribution, it is difficult to determine and/or optimize properties. In addition, miscibility becomes a problem when these liquid polybutadienes, varying significantly in the cis-1,4 content from a higher molecular weight polybutadiene, are used together in blends.
The effects of blends of these high molecular weight/low molecular weight (HMW/LMW) high-cis polybutadiene on the properties of a rubber compound have not been thoroughly investigated. Japanese Patent Publication No. 7-5789, published on the Aug. 6, 1987, disclosed blends of HMW/LMW polybutadiene for use in impact modified polystyrene. Further, Bridgestone Corporation has examined the use of HMW/LMW blends of low-cis polybutadiene for improved wet/snow properties. But nothing has disclosed blends of HMW/LMW blends of low-cis polybutadiene particularly beneficial for use in tread compositions.
Thus, there presently exists a need for a low molecular weight high cis-1,4 polybutadiene having properties suitable for blending with a high molecular weight high-cis polybutadiene, the resultant blend being useful in tire tread compounds.
In part, the present invention relates to a low molecular weight high cis-1,4 polybutadiene and an improved process for polymerization of dienes to a low molecular weight high-cis polybutadiene using either of two neodymium-based catalyst systems. One such catalyst system is referred to throughout as an MAO catalyst system-and comprises: (a) a neodymium-containing compound; (b) an aluminoxane; (c) an organoaluminum hydride compound; and (d) a halide source. While neodymium-based catalyst systems are known to produce high-cis polybutadiene, the use of these four catalyst components to yield a liquid high-cis polybutadiene with a narrow molecular weight distribution was not known. The catalyst system is operational under a variety of conditions.
The resulting polydiene has (a) at least 80 mole percent cis-1,4-butadiene microstructure; (b) a molecular weight distribution of less than 3.1; and (c) a number average molecular weight from about 2000 to about 50,000.
Another such neodymium-based catalyst system disclosed herein, and referred to throughout as a TIBA catalyst system, comprises: (a) a neodymium-containing compound; (b) a trialkyl aluminum compound; (c) an organoaluminum hydride compound; and (d) a halide source. At least 80 weight percent of the resulting polydiene has a molecular weight less than 100,000. In addition, the polydiene has a number average molecular weight less than 35,000; and a ratio of Mp (peak molecular weight) to Mn (number average molecular weight) between about 0.9 and about 2.0. This catalyst system is useful where environmental considerations dictate against the use of hexane-insoluble aluminoxanes in the polymerization process.
Moreover, the present invention also relates to low molecular weight/high-cis polybutadiene blends containing either of these low molecular weight high-cis polybutadienes. In this regard, the present invention relates to a high-cis polybutadiene blend, which comprises: (a) from about 20 to about 80 percent by weight of a first polybutadiene having a number average molecular weight from about 2000 to about 50,000 and a cis-1,4 microstructure content of at least 70 percent; and (b) from about 20 to about 80 percent by weight of a second polybutadiene having a number average molecular weight from about 90,000 to about 300,000, and a cis-1,4 microstructure content of at least 92 percent.
The present invention further relates to a rubber compound, which comprises at least 30 parts by weight of a high cis-1,4-polybutadiene based upon 100 parts by weight rubber wherein a) from about 20 to about 80 weight percent of the high cis-1,4-polybutadiene is a low molecular weight high-cis polybutadiene of the present invention has a molecular weight from about 2,000 to about 50,000 and a cis-1,4 content of at least 70 percent; and b) from about 20 to about 80 weight percent of the high cis-1,4-polybutadiene is a high molecular weight high-cis polybutadiene having a molecular weight from about 90,000 to about 300,000 and a cis-1,4- microstructure of at least 92 percent. Desirably these two fractions of high-cis polybutadiene represent distinct peaks (or modes) in the molecular weight distribution of the high-cis polybutadiene. Desirably the molecular weight distribution of the lower molecular weight peak is from about 1.1 to about 5 and the molecular weight distribution of the higher molecular weight peak in the distribution is from about 1.8 to about 6. These rubber characteristics facilitate achieving a balance of good fracture resistance, snow traction, wet traction, and low rolling resistance.
A process is disclosed below for producing low molecular weight high-cis polybutadiene with a specific molecular weight distribution by polymerizing 1,3-butadiene in the presence of either of two catalyst systems comprising: (a) a neodymium-containing compound; (b) an aluminoxane in the MAO catalyst system or a trialkyl aluminum compound in the TIBA catalyst system; (c) an organoaluminum hydride compound; and (d) a halogen source. Referring particularly to the MAO catalyst system, it differs from JP 8-73515 in that it teaches generally higher amounts of all of the catalyst components relative to the diene, preferred amounts of total aluminum relative to butadiene, and preferred catalyst preparation and aging procedures. One skilled in the art would not anticipate that the molecular weight could be reduced to the extent that liquid polybutadiene was produced without the use of molecular weight regulators. Evaluation of the ratio of the neodymium-containing compound and the total polymers formed in the examples reveals that multiple polymer chains are produced per each neodymium containing compound, so some mechanism of chain termination and chain initiation is taking place which has the same result as chain transfer without broadening the molecular weight distribution.
For the component (a) of the catalyst system, various neodymium-containing compounds can be utilized. It is generally advantageous to employ neodymium-containing compounds that are soluble in a hydrocarbon solvents such as aromatic hydrocarbons, aliphatic hydrocarbons, or cycloaliphatic hydrocarbons. The neodymium-containing compound is desirably soluble in aliphatic or cycloaliphatic solvents at 25 xc2x0 C. to an extent of at least 0.2 mole/liter.
The neodymium in the neodymium-containing compounds can be in various oxidation states. It is preferable to use trivalent neodymium compounds, wherein the neodymium is in the +3 oxidation state. Suitable types of neodymium-containing compounds that can be utilized in the catalyst system include, but are not limited to, neodymium carboxylates, neodymium xcex2-diketonates, neodymium alkoxides and aryloxides, neodymium halides, neodymium pseudo-halides, organoneodymium compounds, and neodymium phosphates or phosphites. The phosphates and phosphites of Group 3B elements are set forth in JP 8-73515, which is hereby incorporated by reference. In this invention, the neodymium carboxylates are preferred over other choices. Disubstituted phosphonates may also be used. Composite salts, in which one of the three ligands is different from the other two, may also be used. A Lewis base may also be present as a stabilizer.
Desirably the neodymium carboxylate is of the formula (R4)3Nd where R4 is a saturated, monounsaturated, or polyunsaturated mono or polycarboxylate of 1 to 20 carbon atoms. Preferably R4 is a straight, branched, or cyclic chain with the carboxyl group bonded to a primary, secondary or tertiary carbon atom. Specific examples of R4 include octanoic acid, 2-ethyl hexanoic acid, oleic acid, stearic acid, versatic acid, neodecanoic acid, benzoic acid, naphthenic acid and bursatic acid (a tradename of a carboxylic acid with the carboxyl group bound to a tertiary carbon atom, manufactured by Shell Chemicals). Some specific examples of suitable neodymium carboxylates include neodymium(III) formate, neodymium(III) acetate, neodymium(III) acrylate, neodymium(III) methacrylate, neodymium(III) valerate, neodymium(III) gluconate, neodymium(III) citrate, neodymium(III) fumarate, neodymium (III) lactate, neodymium(III) maleate, neodymium(III) oxalate, neodymium(III) 2-ethylhexanoate, neodymium(III) neodecanoate, neodymium(III) naphthenate, neodymium(III) stearate, neodymium(III) oleate, neodymium(III) benzoate, and neodymium(III) picolinate.
Desirably the neodymium alkoxide is of the formula (R4O)3Nd where R4 is a linear, branched or cyclic alkyl, or aromatic group of 1,3 or 6 (as appropriate) to 20 carbon atoms. Some specific examples of suitable neodymium alkoxides or aryloxides include neodymium(III) methoxide, neodymium(III) ethoxide, neodymium(III) isopropoxide, neodymium(III) 2-ethylhexoxide, neodymium(III) phenoxide, neodymium(III) nonylphenoxide, and neodymium(III) naphthoxide.
Examples of the xcex2-diketone complexes include acetylacetone, benzoylacetone, propionnitrileacetone, valerylacetone and ethyl acetylacetone complexes of the metal.
Examples of the phosphates or phosphites of the metal include bis(2-ethylhexyl)phosphate, bis(1-methylbutyl)phosphate, bis(p-nonylphenyl)phosphate, bis(polyethylene glycol-p-nonylphenyl)phosphate, (1-methylheptyl)(2-ethylhexyl)phosphate, (2-ethylhexyl)(p-nonylphenyl)phosphate, 2-ethylhexyl phosphonate mono-2-ethylhexyl, 2-ethylhexyl phosphonate mono-p-nonylphenyl, bis(2-ethylhexyl)phosphinate, bis(1-methylbutyl)phosphinate, bis(p-nonylphenyl)phosphinate, (1-methylheptyl)(2-ethylhexyl)phosphinate, (2-ethylhexyl)(p-nonylphenyl)phosphinate, and other salts.
The component (b) of the catalyst system is an alkylaluminoxane in the MAO catalyst system or, in the alternative, a trialkyl aluminum compound in the TIBA catalyst system. These are well known to the art. Examples of the alkylaluminoxane component include compounds with the formulas
R52xe2x80x94Alxe2x80x94(Oxe2x80x94Al)mxe2x80x94Oxe2x80x94Alxe2x80x94R52
or

where m is an integer greater than 2 and preferably greater than or equal to 5 and most preferably 2, 5, or 10 to 100. R5 is a hydrocarbon group of 1 to 6 carbon atoms such as methyl, ethyl, propyl and butyl groups. Preferably R5 is methyl or ethyl. Preferred alkylaluminoxanes are methyl aluminoxane, ethyl aluminoxane, propyl aluminoxane, butyl aluminoxane, and isobutyl aluminoxane. It should be noted that moles or millimoles of aluminoxanes as used in this application refers to moles of Alxe2x80x94R5 rather than moles of the oligomer or cyclic compound. This is conventional in the art of catalysis with aluminoxanes.
Suitable examples of trialkyl aluminum compounds for practice in the present invention include trimethyl aluminum, triethyl aluminum, tri (iso- or n-) propyl aluminum, tri-isobutyl or tri-n-butyl aluminum, etc. In the preferred embodiment, the trialkyl aluminum compound is tri-isobutyl aluminum.
The component (c) of the catalyst system is an organoaluminum hydride compound. As used herein, the term xe2x80x9can organoaluminum hydride compoundxe2x80x9d refers to any aluminum compound containing at least one covalent aluminum-carbon bond and at least one covalent aluminum-hydrogen bond. It is generally advantageous to employ organoaluminum hydride compounds that are soluble in the hydrocarbon polymerization medium. Thus suitable types of organoaluminum hydride compounds that can be utilized in the catalyst system include, but are not limited to, dihydrocarbylaluminum hydride compounds and hydrocarbylaluminum dihydride compounds, which are represented by the formula AlHnR3xe2x88x92n (n=1 or 2), wherein each R, which may be the same or different, is selected from the group consisting of alkyl, cycloalkyl, aryl, aralkyl, alkaryl, and allyl groups; each group preferably contains from 1 or the appropriate minimum number of carbon atoms to form this group to 20 carbon atoms. Dihydrocarbylaluminum hydride compounds are generally preferred.
Some specific examples of suitable organoaluminum hydride compounds that can be utilized in the catalyst system are: diethylaluminum hydride, di-n-propylaluminum hydride, diisopropylaluminum hydride, di-n-butylaluminum hydride, diisobutylaluminum hydride, di-n-octylaluminum hydride, diphenylaluminum hydride, di-p-tolylaluminum hydride, dibenzylaluminum hydride, phenylethylaluminum hydride, phenyl-n-propylaluminum hydride, phenylisopropylaluminum hydride, phenyl-n-butylaluminum hydride, phenylisobutylaluminum hydride, phenyl-n-octylaluminum hydride, p-tolylethylaluminum hydride, p-tolyl-n-propylaluminum hydride, p-tolylisopropylaluminum hydride, p-tolyl-n-butylaluminum hydride, p-tolylisobutylaluminum hydride, p-tolyl-n-octylaluminum hydride, benzylethylaluminum hydride, benzyl-n-propylaluminum hydride, benzylisopropylaluminum hydride, benzyl-n-butylaluminum hydride, benzylisobutylaluminum hydride, and benzyl-n-octylaluminum hydride and other organoaluminum monohydrides. Also included are ethylaluminum dihydride, n-propylaluminum dihydride, isopropylaluminum dihydride, n-butylaluminum dihydride, isobutylaluminum dihydride, and n-octylaluminum dihydride and other organoaluminum dihydrides. Mixtures of the above organoaluminum hydride compounds may also be utilized.
The catalyst system for low molecular weight polybutadienes further comprises a halide source as the component (d). The halide source can be either 1) a halogenated organic (hydrocarbon) compound, 2) a halogenated alkyl metal compound such as halogenated alkyl aluminum, halogenated alkyl magnesium, compounds with silicon chloride bonds such as trialkylsilicon chloride, or halogenated alkyl zinc; or 3) a metal halide such as magnesium chloride, tin chloride, or silicon tetrachloride. The halogen of the halogen source can be chlorine, bromine, fluorine or iodine. The halogen source may have one or more halogen atoms per molecule. If it is an organic halogen containing compound, it desirably has from about 1, 2, or 3 to about 15 or 20 carbon atoms. The organic compound may be linear, branched, cyclic, aromatic, etc. It is preferred to have an organic compound with a labile halogen atom. Examples of such compounds include tertiary carbons, aromatic carbons, and allylic carbons. Preferred halogen sources include benzoyl chloride, benzyl chloride, benzylidene chloride, allyl chloride, propionyl chloride, allyl chloride, and t-butyl chloride; the brominated versions of the above compounds; methyl chloroformate or methyl bromoformate; and chloro-di-phenylmethane or chloro-tri-phenylmethane; and the like. The halogenated alkyl aluminum compound desirably has the structure AlXnR6(3xe2x88x92n) where X is the above listed halogen, R6 is a hydrocarbon with from 1 to 8 carbon atoms, and n is 1 or 2. Examples of said halogenated alkyl aluminum include dimethyl aluminum chloride, diethyl aluminum chloride, diethyl aluminum bromide, diethyl aluminum iodide, diethyl aluminum fluoride, di-n-propyl aluminum chloride, di-n-butyl aluminum chloride, di-isobutyl aluminum chloride, methyl aluminum-di-chloride, ethyl aluminum-di-chloride isobutyl aluminum-di-chloride, sesquimethyl aluminum chloride, sesquiethyl aluminum chloride, sesqui-isobutyl aluminum chloride and mixtures thereof. The metal halides include aluminum trichloride, aluminum tribromide, aluminum triiodide, aluminum trifluoride etc. and mixtures thereof.
The catalyst system comprises the above-described four components (a), (b), (c), and (d) as the main components. In addition to the four catalyst components (a), (b), (c), and (d), other catalyst components such as other organometallic compounds or Lewis bases, which are known in the art, can be added.
The catalyst system has very high catalytic activity over a wide range of total catalyst concentrations and catalyst component ratios. The four catalyst components (a), (b), (c), and (d) apparently interact to form the active catalyst species. Accordingly, the optimum concentration for any one catalyst component depends on the concentrations of the other catalyst components. While polymerization will occur over a wide range of catalyst concentrations and catalyst component ratios, polymers having the most desirable properties are obtained with a narrow range of catalyst concentrations and catalyst component ratios.
The molar ratio of the aluminoxane to the neodymium-containing compound (Al/Nd) in the MAO catalyst system can be varied from about 10 to about 500. However, a more preferred range of Al/Nd molar ratio is from about 40 or 50 to about 200, and the most preferred range is from about 75 to about 150. The molar amount of aluminoxane is the number of moles of Alxe2x80x94R5 units rather than the moles of the oligomer or cyclic aluminoxane. This is consistent with the effectiveness of aluminoxane in this type of catalyst system.
The molar ratio of the trialkyl aluminum compound to the neodymium-containing compound (TIBA/Nd) in the TIBA catalyst system can be varied from about 30 to about 200. However, a more preferred range of TIBA/Nd molar ratio is from about 30 to about 100, and the most preferred range is from about 40 to about 60.
The molar ratio of the organoaluminum hydride compound to the neodymium-containing compound (Al/Nd) in the MAO catalyst system can be varied from about 10 to about 100. The more preferred range of Al/Nd molar ratio is from about 10 to about 50 or 60, and the most preferred range is from about 15 to about 25 or 30.
The molar ratio of the organoaluminum hydride compound to the neodymium-containing compound (Al/Nd) in the TIBA catalyst system can be varied from about 1 to about 100. The more preferred range of Al/Nd molar ratio is from about 10 to about 50, and the most preferred range is from about 10 to about 30.
The molar ratio of the halogen source to the neodymium-containing compound (Halogen/Nd) in the MAO catalyst system can be varied from about 1 to about 15, with the more preferred range of halogen/Nd molar ratio being from about 2 to about 8 and the most preferred range being from about 2 to about 6.
Referring now to the TIBA catalyst system, the molar ratio of the halogen source to the neodymium-containing compound (Halogen/Nd) can be varied from about 2 to about 16, with the more preferred range of about 2 to about 12, and a most preferred range of about 4 to about 12.
The total catalyst concentration in the polymerization mass depends on such factors as the purity of the components, the polymerization rate and conversion desired, the polymerization temperature, and the like. Accordingly, specific total catalyst concentrations cannot be definitively set forth except to say that catalytically effective amounts of the respective catalyst components should be used.
Generally, the amount of the neodymium-containing compound used in the MAO catalyst system can be varied from about 0.1 to about 3 mmol per 100 g of 1,3-butadiene, with a more preferred range being from about 0.2 to about 2 mmol per 100 g of 1,3-butadiene and the most preferred range being from about 0.3 to about 1.5 mmol per 100 g of 1,3-butadiene.
As to the TIBA catalyst system, the amount of neodymium-containing compound used can be varied from about 0.1 to about 1.0 mmol per 100 g of 1,3-butadiene, with a more preferred range being about 0.2 to about 0.6 mmol per 100 g of 1,3-butadiene, and a most preferred range of about 0.2 to 0.4 mmol per 100 g 1,3-butadiene.
Certain specific total catalyst concentrations and catalyst component ratios which produce polymers having desired molecular weight and molecular weight distributions will be illustrated in the examples given to explain the teachings of the present invention.
The diene component is preferably butadiene but may include other nonhalogenated conjugated dienes with from 4 to 8 carbon atoms. Examples of other dienes are isoprene, 1,3-hexadiene, etc. It is desirable in the preferred embodiment that at least 80 mole percent of the repeat units of the polymer be derived from polymerizing butadiene, more desirably at least 85 mole percent are from butadiene and preferably at least 90 or 95 mole percent are from polymerizing butadiene.
Referring particularly to the MAO catalyst system, it has been discovered that relatively large amounts of aluminum are required to achieve the high-cis liquid polydienes. If the ratios set forth in this invention are maintained between various catalyst components, it is not critical which component or components of the catalyst system are the sources of the necessary aluminum. A desirable amount of total aluminum (from all of the catalyst sources) is from about 0.01 to about 0.1 mole of aluminum per mole of diene (e.g. butadiene).
It is desirable that the resulting polymer using either the MAO or TIBA catalyst system has at least 75 or 80 mole percent repeat units of the cis-1,4 microstructure, more desirably at least 80 mole percent and preferably at least 85 mole percent.
It is desirable that the low molecular weight polymer prepared in accordance with the MAO catalyst system has a number average molecular weight from about 2000 to about 50,000, and more desirably from about 4,000 to about 25,000. It is desirable that the resulting polymer have a molecular weight distribution (defined as the weight average molecular weight (Mw) divided by the number average molecular weight (Mn)) of less than 3.1 and preferably from about 1.2 to about 2.2.
Referring to the low molecular weight polymer of the TIBA catalyst system, preferably at least 80 weight percent of that polymer (by GPC) has a molecular weight less than 100,000, and more desirably, at least 85 weight percent of that polymer has a molecular weight less than 100,000. In the preferred embodiment at least 88 weight percent of that polymer has a molecular weight less than 100,000. Moreover, the resilient polymer preferably has a number average molecular weight (Mn) less than 35,000, more preferably less than 30,000, and most preferably less than 25,000. The ratio Mp/Mn of the polymer preferably varies from about 0.9 to about 2.0.
The catalyst components may be introduced into the polymerization system in several different ways. They may be added in either a stepwise or simultaneous manner. It is desirable to add the halide source as the last component of the catalyst system and preferably after a portion of the total diene has been added. The order in which the components are added in a stepwise manner is not critical to achieve polymerization but may affect the number average molecular weight of the polymer. With respect to the MAO catalyst system, the components are preferably added in the order of the 1) part of the total diene, 2) aluminoxane, 3) neodymium-containing compound, and 4) organoaluminum hydride. As to the TIBA catalyst system, there is no preferred order for the addition of the components, but, again, the halide source must be added last in the polymerization. It is optional to age the reactants for a few seconds to minutes prior to the addition of the halide source. The catalyst components may be premixed outside the polymerization system at an appropriate temperature (e.g., from about 10xc2x0 C. to about 90xc2x0 C.), following by the addition of the catalysts to the polymerization system or the catalysts may be mixed in the polymerization reactor. The amount of diene, e.g.1,3-butadiene monomer, which is desirably added before the halide source, can range from about 10 to about 100 moles per mole of the neodymium-containing compound, and preferably should be from about 10 to about 50 moles per mole of the neodymium-containing compound.
It has been observed by others that increased aging time (after adding the halide source) for these types of initiators usually increases the activity of the catalyst. While preparing the examples it was observed that shorter catalyst aging times generate more polymer chains and consequently, reduce the molecular weight of the resulting polymers. Thus aging time is a compromise between catalyst activity and catalyst efficiency when producing low molecular weight polymers. It is desirable to age the catalyst (after adding the halide source) less than 30 minutes at a temperature of less than 50xc2x0 C. and more desirably less than 10 minutes at from about 10 to about 50xc2x0 C. after mixing all the catalyst components and part of the diene.
When a catalyst solution is prepared outside the polymerization system, the organic solvent usable for the catalyst component solution may be selected from aromatic hydrocarbons, aliphatic hydrocarbons and cycloaliphatic hydrocarbons, and mixtures of two or more of the above-mentioned hydrocarbons. Preferably, the organic solvent consists of at least one selected from benzene, toluene, xylene, hexane, heptane and cyclohexane.
The polymerization of 1,3-butadiene via this process is carried out in an organic solvent as the diluent. In such cases, a solution polymerization system may be employed in which both the monomer and the polymer formed are soluble in the polymerization medium. Additional organic solvent may be added. It may be the same as or different from the organic solvent contained in the catalyst component solutions. It is normally desirable to select an organic solvent that is inert with respect to the catalyst system. Suitable types of organic solvents include, but are not limited to, aliphatic, cycloaliphatic, and aromatic hydrocarbons. Some representative examples of suitable aliphatic solvents include n-pentane, n-hexane, n-heptane, n-octane, n-nonane, n-decane, isopentane, isohexanes, isopentanes, isooctanes, 2,2-dimethylbutane, petroleum ether, kerosene, petroleum spirits, and the like. Some representative examples of suitable cycloaliphatic solvents include cyclopentane, cyclohexane, methylcyclopentane, methylcyclohexane, and the like. Some representative examples of suitable aromatic solvents include benzene, toluene, xylenes, ethylbenzene, diethylbenzene, mesitylene, and the like. Commercial mixtures of the above hydrocarbons may also be used. For environmental reasons, aliphatic and cycloaliphatic solvents are highly preferred. The concentration of the 1,3-butadiene monomer is not limited to a special range. However, generally, it is preferable that the concentration of the diene, e.g. 1,3-butadiene, in the polymerization reaction mixture is in a range of from about 3% to about 80% by weight, but a more preferred range is from about 5% to about 50% by weight, and the most preferred range is from about 10% to about 30% by weight.
A major benefit of the MAO catalyst system process for forming low molecular weight (liquid) high-cis polybutadiene is the lack of added molecular weight regulators, which often decrease the rate of polymerization and lead to broad molecular weight distributions. The process is not, however, free of chain transfer reactions as a close examination of the experimental data will demonstrate that more moles of polymer are generated than can be explained based on the moles of the initiator.
In accordance with either the MAO catalyst system process or the TIBA catalyst system process, the polymerization of 1,3-butadiene may be carried out as a batch process, on a semi-continuous basis, or on a continuous basis. In any case, the polymerization is conducted under anaerobic conditions using an inert protective gas such as nitrogen, argon or helium, with moderate to vigorous agitation. The polymerization temperature may vary widely from a low temperature, such as 10xc2x0 C. or below, to a high temperature such as 130xc2x0 C. or above, with a preferred temperature range being from about 20xc2x0 C. to about 90xc2x0 C. The heat of polymerization may be removed by external cooling, cooling by evaporation of the 1,3-butadiene monomer or the solvent, or a combination of the two methods. Although the polymerization pressure may vary widely, the preferred pressure range is from about 1 atmosphere to about 10 atmospheres.
After reaching a desired conversion, the polymerization reaction can be stopped by addition of a known polymerization terminator into the polymerization system to inactivate the catalyst system, followed by the conventional steps of desolventization and drying as are typically employed by and are known to those skilled in the art of conjugated diene polymerization. Typically, the terminator employed to inactivate the catalyst system is a protic compound that includes, but is not limited to, an alcohol, a carboxylic acid, an inorganic acid, and water or a combination thereof. An antioxidant such as 2,6-di-tert-butyl-4-methylphenol may be added along with, before or after addition of, the terminator. The amount of the antioxidant employed is usually in the range of 0.2% to 1% by weight of the polymer product.
The polymer may be isolated from the solvents by evaporation (forced or natural) of the solvent from the polymer cement. At low molecular weights, polybutadiene is a liquid and its isolation by coagulation is difficult. Alternatively, the solution of liquid polybutadiene can be blended with a solution of other higher molecular weight rubbers/polymers and then desolventized. This procedure is practical when the low molecular weight polybutadiene is going to be blended before use.
The low molecular weight high-cis polybutadiene product produced by the above process has many applications. It can be blended with various rubbers in order to improve their properties. For example, it can be incorporated into elastomers in order to improve or modify their viscoelastic properties (such as Gxe2x80x2 and tan xcex4) at a particular temperature. It has been possible to increase the snow and wet traction of a rubber blend with this low molecular weight high cis-1,4 polybutadiene. These properties are generally correlated with a lower storage modulus (Gxe2x80x2) at xe2x88x9220xc2x0 C. and a higher tan xcex4 at 0xc2x0 C. respectively when these properties are measured at 1 Hz and small strains.
More particularly, the addition of a blend of this low molecular weight high-cis polybutadiene and a high molecular weight high-cis to a rubber compound has been found to improve properties such as fracture resistance, snow traction, wet traction, and rolling resistance. It is beneficial if the blend has a molecular weight distribution which has at least two modes, with a first mode having a maximum between a molecular weight of 2000 and 50,000 and a second mode having a maximum between a molecular weight of 90,000 and 300,000. It is particularly advantageous that both the high and low molecular weight polybutadienes have similar amounts of cis-1,4 repeating units as this results in enhanced compatibility of the two polymers over blends where the cis-1,4 content varies significantly between the high and low molecular weight polymers. While the high and low molecular weight polymers are generally characterized as two different materials, which are separately prepared, it is specifically acknowledged that due to the similarity in the catalyst systems used to prepare the high and low molecular weight polymers it is beneficial to make both the high and low molecular weight fractions in the same reactor or plant and/or blend them before isolating the polymers from their polymerization media.
The benefit from using a blend of high and low molecular weight high-cis polybutadiene is not limited to rubber compounds where the high and low molecular weight polybutadiene is 100% of the rubber component. In fact these polymers are easily blended with conventional rubbers used in tires. A blend of high and low molecular weight high-cis polybutadiene with one or more other rubbers may be optimized for a total balance of tire or tire tread properties. The weight percent of the high molecular weight high-cis polybutadiene is desirably from about 20 to about 80 percent, and is more desirably from about 25 to about 75 percent, and is preferably from about 30 to about 70 percent of the blend of high and low molecular weight high-cis polybutadienes.
The high molecular weight polybutadiene desirably has a molecular weight or a number average molecular weight from about 90,000 to about 300,000, more desirably from about 150,000 or 200,000 to about 280,000. By using molecular weight alternatively to number average molecular weight it is intended to provide alternative but nearly equivalent options in the claims. Desirably the molecular weight distribution (MWD or Mw/Mn) is from about 1.8 or 2.0 to about 6.0 and more desirably from about 1.8 to about 3.2. Desirably the high molecular weight polybutadiene has a cis-1,4 content of at least 92% and preferably at least 94%. Polymers of this type are commercially available or can be prepared by using catalyst systems based on nickel or neodymium carboxylates, trialkyl aluminum, and a compound with a labile halide. The weight percent of the high molecular weight high-cis polybutadiene is desirably from about 20 to about 80, and is more desirably from about 25 to about 75 percent, and is preferably from about 30 to about 70 percent of the blend of high and low molecular weight high-cis polybutadiene. The polybutadiene can tolerate small amounts of comonomers, e.g. less than1, 5, or 10 percent of another diene or another monomer, so long as the high molecular weight polybutadiene is compatible with the low molecular weight butadiene and other rubbers in the blend. By compatibility it is meant that the polymers can be mixed thoroughly without macroscopic phase separation.
If a polymer is described as having a number average molecular weight in a certain range then only the number average needs to fall in that range and the polymer may and probably will include a small fraction of polymer chains having a molecular weight outside the specified range.
The low molecular weight polybutadiene desirably has a molecular weight or number average molecular weight from about 2,000 or 4,000 to about 40,000 or 50,000, and is more desirably from about 5,000 to about 20,000 or 25,000. Desirably the molecular weight distribution is from about 1.1 to about 5, and is more desirably from about 1.2 or to about 2.2 . Desirably the cis-1,4 content is at least 70%, more desirably from about 70 to about 97%, preferably at least 85%, and is more preferably from about 85 to about 97%. Desirably the low molecular weight high-cis polybutadiene is from about 20 to about 80 weight percent of the blend of high and low molecular weight high-cis polybutadiene, and is more desirably from about 25 to about 75 weight percent and is preferably from about 30 to about 70 weight percent of the blend. The polybutadiene can tolerate small amounts of comonomers, e.g. less than 1, 5, or 10 percent of another diene or another monomer, so long as the low molecular weight polybutadiene is compatible with the high molecular weight polybutadiene and other rubbers in the blend.
The rubber compounds considered here generally include reinforcing fillers, oil extenders (plasticizers), and curatives. The reinforcing fillers can include carbon black, and silica (optionally with a silane treatment or a silane coupling agent) etc. Desirably the amount of filler is from about 10 to about 100 phr and preferably from about 30 to about 80 parts by weight per one hundred parts by weight of rubber (phr). A plasticizing agent, such as a paraffinic, aromatic, or naphthenic oil, may be used, desirably in an amount from about 0 to about 30 phr and preferably from about 0 to about 15 phr. The curatives are generally sulfur and one or more accelerators although other curatives may be used. The sulfur (if used as a curing agent) is generally present from about 0.5 to about 5 phr. The accelerators are generally used from about 0.5 to about 5 phr.
The rubber compounds are desirably used in a pneumatic tire and more desirably in a tire tread where the material characteristics of the rubber have a direct impact on snow traction, wet traction, and rolling resistance. They may be used in other applications where a certain combination of elastic modulus and hysteretic properties would be of some benefit.
The practice of the present invention is further illustrated by reference to the following examples, which however, should not be construed as limiting the scope of the invention. Parts and percentages shown in the examples are by weight unless otherwise indicated.
General Considerations
Molecular weight and molecular weight distribution were obtained by using a GPC instrument equipped with two Tosoh GMHXL (30 cm) columns connected in line. THF was used as carrier solvent, with a flow rate of 1.0 mL/min. The instrument was universally calibrated with polystyrene standards and Mark-Howick constants for high-cis polybutadienes. Microstructures were obtained by FT-IR measurements. Spectra of CS2 solutions of polymers (0.5 w/v %) were obtained, and the microstructures were calculated by Morello""s method